Project Summary The development of isoform-selective histone deacetylase (HDAC) radiotracers will fill critical knowledge gaps that exist in our understanding of HDAC function in the human brain. HDAC6 is primary among the isoforms of HDAC that, to date, have exhibited strong therapeutic potential. Compelling preclinical evidence supports HDAC6 inhibition for the treatment of depression and neurodegenerative diseases and translational tools such as a positron emission tomography radiotracer targeting HDAC6 will allow us to assess the relevance of preclinical findings in the human brain. HDAC6 acts not by removing acetyl groups from histones as its name might suggest, but instead by regulating the acetylation state of several non-histone proteins including ?- tubulin, cortactin, tau, HSP90 and ?-catenin. Inhibitors that selectively engage HDAC6 over the other ten isoforms of HDAC have been developed by our team and by others in the field and provide the foundation for PET radiotracer development. Using known HDAC6 inhibitor scaffolds, we will design and synthesize a library of HDAC6 inhibitors that can be readily labeled with a positron-emitting isotope (carbon-11 or fluorine-18). The library will undergo rigorous physiochemical and biochemical profiling including assays that evaluate HDAC- isoform selectivity and efficacy in cultured human neurons and that predict blood-brain barrier penetration. Compounds prioritized through these assays will be radiolabeled and evaluated in rodents for brain uptake, pharmacokinetics and specific (saturable) binding. Promising in vivo imaging results will be validated using HDAC6 knock-out mouse imaging. Finally to set the stage for translation to human HDAC6 imaging (which our team has accomplished for class-I HDAC imaging), we will evaluate priority compounds in non-human primates to determine regional brain binding using full metabolite-corrected arterial blood input function correction and kinetic modeling. By the end of the grant project period, our team will be poised to measure the density-distribution of HDAC6 in the human brain over the course of natural heterogeneity (e.g. age and sex differences) and in patients with HDAC6-implicated brain disorders.